A disk drive is a data storage device that stores digital data in tracks on the surface of a data storage disk. Data is read from or written to a track of the disk using a transducer, which includes a read element and a write element, that is held close to the track while the disk spins about its center at a substantially constant angular velocity. To properly locate the transducer near the desired track during a read or write operation, a closed-loop servo scheme is generally implemented. The servo scheme uses servo data read from the disk surface to align the transducer with the desired track. The servo data is generally written to the disk using a servo track writer (STW).
In an ideal disk drive system, the tracks of the data storage disk are non-perturbed circles situated about the center of the disk. As such, each of these ideal tracks includes a track centerline that is located at a known constant radius from the disk center. In an actual system, however, it is difficult to write non-perturbed circular tracks to the data storage disk. That is, due to certain problems, such as vibration, bearing defects, inaccuracies in the STW and disk clamp slippage, tracks are generally written differently from the ideal non-perturbed circular track shape. Positioning errors created by the perturbed nature of these tracks are known as written-in repetitive runout (STW—RRO).
The perturbed shape of these tracks complicates the transducer positioning function during read and write operations because the servo system needs to continuously reposition the transducer during track following to keep up with the constantly changing radius of the track centerline with respect to the center of the spinning disk. Furthermore, the perturbed shape of these tracks can result in problems such as track squeeze and track misregistration errors during read and write operations.
In order to reduce such problems, disk drive manufacturers have developed techniques to measure the STW—RRO, so that compensation values (also known as embedded runout correction values or ERC values) may be generated and used to position the transducer along an ideal track centerline. Examples of such techniques may be found in U.S. Pat. No. 4,412,165 to Case et al. entitled “Sampled Servo Position Control System”, U.S. Pat. No. 6,115,203 to Ho et al. entitled “Efficient Drive-Level Estimation of Written-In Servo Position Error”, and U.S. patent application Ser. No. 09/753,969 filed Jan. 2, 2001 and entitled “Method and Apparatus for the Enhancement of Embedded Runout Correction in a Disk Drive”, all of which are incorporated herein by reference.
In some prior systems, disk drive manufacturers have resorted to developing compensation values for each and every sector of each and every track of a disk drive. That is, disk drive manufacturers have tried to develop compensation values so that each track appears to be nearly an ideally written track. However, as those skilled in the art will readily appreciate, developing compensation values associated with each and every sector of each and every track is extremely time consuming and, therefore, increases manufacturing times. Furthermore, valuable manufacturing time may be wasted on tracks that have been well-written.
In an attempt to reduce manufacturing times as compared to the above, disk drive manufacturers have developed a technique to correct only the most poorly-written tracks for a production run of disk drives, instead of all of the tracks of each drive. In such technique, an embedded runout correction (ERC) threshold is set for all disk drives in a production run. The ERC threshold is used to determine which tracks are to be corrected in a particular production run of disk drives.
More specifically, the position error due to repeatable runout (PES—RRO) is measured by track following and averaging the position error from the servo bursts in each servo sector associated with the track for multiple revolutions of the disk (e.g., 25 revolutions). As will be understood by those skilled in the art, the position error is averaged for multiple revolutions of the disk, so that the affects of non-repeatable runout may be averaged out.
If the absolute value of the average PES—RRO for any servo sector in the track exceeds the ERC threshold, the track is corrected. That is, compensation values are determined for all of the servo sectors of that track. However, if the absolute value of the average PES—RRO for all of the servo sectors in the track are less than the threshold, the track is not corrected.
In a similar technique, in which an attempt is made to reduce the number of revolutions required to measure the position error due to repeatable runout (PES—RRO), two ERC thresholds are used, namely, a high-sensitivity ERC threshold (i.e., a low value) and a low-sensitivity ERC threshold (i.e., a high value). Importantly, both the high-sensitivity ERC threshold and the low-sensitivity ERC threshold are set for all disk drives in a production run.
In the case of using high-sensitivity and low-sensitivity thresholds, the position error due to repeatable runout (PES—RRO) is measured by track following and averaging the position error from the servo bursts in each servo sector associated with the track for a relatively small number of revolutions of the disk (e.g., two revolutions). The absolute value of the average PES—RRO for each servo sector in the track is then compared to the high-sensitivity ERC threshold. If the absolute value of the average PES—RRO for each servo sector is less than the high-sensitivity ERC threshold, the track is not corrected and the next track is tested.
If, instead, the absolute value of the average PES—RRO for at least one of the servo sectors associated with the track is greater than the high-sensitivity ERC threshold, the position error from the servo bursts in each servo sector associated with the track is measured for additional revolutions of the disk (e.g., two additional revolutions) and is averaged into the existing average PES—RRO. The absolute value of the average PES—RRO (in this example, for four revolutions) for each servo sector in the track is then compared to the low-sensitivity threshold. If the absolute value of the average PES—RRO for each servo sector is less than the low-sensitivity threshold, the track is not corrected and the next track is tested. If, instead, the absolute value of the average PES—RRO for any one of the servo sectors is greater than the low-sensitivity threshold, the track is corrected. That is, compensation values are determined for all of the servo sectors of that track. This may include additional revolutions of position error averaging.
The two thresholds are used because, after only a small number of revolutions (e.g., two revolutions), a high amount of non-repeatable energy (which has not been averaged away) may affect the measurements of the PES—RRO. Thus, a well-written track might look like a poorly-written track, while a poorly-written track may look like a well-written track. Setting both a high-sensitivity ERC threshold and a low-sensitivity ERC threshold is believed to more accurately identify tracks which should be corrected, while reducing the overall time to make such a determination.
In yet another technique, a high-sensitivity ERC threshold and a low-sensitivity ERC threshold are set for both the read position and the write position of the read element. In such case, the high-sensitivity ERC threshold for the read position, the high-sensitivity ERC threshold for the write position, the low-sensitivity ERC threshold for the read position and the low-sensitivity ERC threshold for the write position are set for all disk drives in a production run.
As will be understood by those skilled in the art, the read element and the write element are offset from one another, and this offset varies across the stroke of the actuator arm due to head skew. Because write-to-write track misregistration is both a more common and more severe problem than write-to-read misregistration, the ERC thresholds are set to a much lower trigger point when the read element is in its write position as opposed to when the read element is in its read position. This is because the write element is generally between one and a half and two times wider than the read element and is slightly narrower than the width of a track. Thus, write position errors will tend to cause encroachment problems and, hence, data destruction. Accordingly, tolerances when the read element is in its read position are generally greater than tolerances when the read element is in its write position.
In general, the process of selecting and correcting poorly-written tracks takes place in a self-test procedure. Each drive is required to complete its self-test procedure within a predetermined period of time. If a drive does not complete its self-test procedure within the predetermined period of time, it is considered to have failed the self-test procedure ard is discarded. Thus, the setting of ERC thresholds is a compromise between the number of tracks being corrected and self-test time.
Typically, thresholds used to select which poorly-written tracks will be corrected are generally set at a very high level, so that only the worst tracks in a production run of drives are corrected and so that very few drives fail self-test due to the embedded runout correction procedure. Due to the thresholds being set on a production run of drives basis, most drives will only use a very small percentage of the self-test time to improve their poorly-written tracks, even though further time is available to make improvements to the tracks of such drives.
Accordingly, it would be desirable to make more efficient use of self-test time, so that additional tracks may be improved through the embedded runout correction procedure. Furthermore, it would be desirable to develop a scheme of automatically selecting one or more ERC thresholds on a drive-by-drive basis, instead of a product-by-product basis.